Structural materials today need to operate at higher temperatures. Most ceramic materials have good long-term stability against creep and chemical attack at temperatures above the operating range for current alloys. Because of ceramic's low fracture energies, however, ceramics are subject to catastrophic failure. Even relatively small defects can start propagation of cracks that can catastrophically propagate through the ceramic component. Therefore, measures for improving their fracture toughness, i.e., toughening ceramics without sacrificing their excellent properties, are sought after.
One way to toughen ceramics is to have fiber-reinforced ceramic composites. The fiber-reinforced ceramics generally are called ceramic matrix composites, or CMCs. Fiber-reinforced ceramic composites possess higher temperature capability and lighter weight than those of the currently used superalloys. As a result, they are being considered for use in aircraft and power generator systems. In these potential applications, fiber-reinforced ceramic composites are subjected to severe thermal and mechanical conditions. Although the fiber-reinforced composites will probably be designed to be used below their matrix cracking stress, accidental over stressing, either thermally as a result of a thermal shock or mechanically during a foreign object impact, can hardly be avoided.
Cracks will be generated in the fiber-reinforced composite matrix when the composite is subjected to a higher stress than its matrix cracking stress. Such cracks will stay open even if the operating stress is subsequently reduced to a value below the matrix cracking stress, exposing coatings and/or fibers to the environment. As a result, the existence of cracks in fiber-reinforced composite matrices will affect the performance and durability of the composites, especially if the cracks are through the thickness of the composites.
These cracks can serve as a fast path for the transport of environmental gaseous phases into the composite. Oxygen can diffuse very rapidly through even extremely small cracks in the matrix. The fibers and any coating that may be on the fiber can oxidize by oxygen diffusing through the crack. Oxygen reacts with the fiber coating and eventually the fiber, causing local bonding between the fiber and matrix. Fiber failure will initiate at this bonded location because of the resultant stress concentration and fiber degradation. The same occurs when the fiber ends are exposed. This process continues until the remaining fibers are unable to carry the load, and the composite fails at a stress appreciably less than the ultimate strength. The composite also loses its tough behavior because of the strong bonding between the fiber and the matrix. Thus, a serious problem limiting the life of ceramic matrix composites is the oxidation of the fiber coating followed by oxidation of the fiber at the base of the crack.
Generally, carbon or boron nitride have been the materials of choice for fiber coatings used in ceramic matrix composites for high temperature use. They provide for fiber protection during composite processing and they also provide a weak fiber to matrix interface, which increases toughness of the composite by fiber pullout or fiber-matrix debonding. However, carbon oxidizes in all oxygen-containing environments, and boron nitride is susceptible to oxidation and subsequent volatilization at high temperatures, particularly in atmospheres with high partial pressures of water vapor, such as products of fuel combustion as in gas turbine engines. Oxidation and volatilization of the boron nitride coatings lead to bonding of the fiber and matrix, thereby yielding a brittle composite. It is thus desirable to have a fiber coating which exhibits fiber-matrix debonding, but is more resistant to oxidation and high water content environments.
The ceramic matrix composites of interest for engine applications have focused on carbon-carbon composites, having a carbon matrix with carbon fibers, and silicon carbide composites, which have a silicon carbide matrix with silicon carbide fibers, the fibers usually being coated. An important limitation to the use of carbonized structural materials is their susceptibility to oxidation in high temperature, oxidizing environments. Oxygen attacks the surface of the carbonized material and seeps into the pores of interstices that invariably are present, oxidizing the surfaces of the pores and continuously weakening the material. The oxidizing atmosphere reaching the fibers, carbon and graphite fibers, seriously degrades the composite structure. An approach to overcome the oxidation of carbon-carbon composites has been the use of glass-formers as oxidation inhibitors. The glass-formers are used as coatings surrounding the outer carbon-carbon composite. In spite of the advances that have been made in carbon-carbon composites, there is still a demand for improved ceramic composites with higher temperature and mechanical capability.
Silicon carbide-silicon carbide composites are silicon carbide fibers in a silicon carbide matrix. A method of making silicon carbide composites is the use of chemical vapor infiltration. Here, layers of cloth made of the fiber material are coated with boron nitride by chemical vapor infiltration. This takes about one day to deposit about 0.5 micrometers of coating material. The layers of cloth are then coated with silicon carbide by chemical vapor infiltration for about 10 to 20 days. An approach to overcome the oxidation of silicon carbide composites has been the use of an oxygen-scavenging sealant-forming region in intimate contact with the ceramic fibers and a debonding layer, as described in U.S. Pat. No. 5,094,901.
There is a need for an improved ceramic matrix composite that successfully protects reinforcing fibers from oxidation and water-containing environments. There is a further need for a method of making a ceramic matrix composite that takes less time than the chemical vapor infiltration methods for silicon carbide-silicon carbide composites and carbon-carbon composites. There is also a need for a method to make silicon--silicon carbide matrix composites and articles made from molten silicon infiltration that provides protection in dry and water vapor-containing environments at high temperatures, greater than about 600.degree. C.